Hazard detection systems can utilise a variety of sensors to detect hazards, including smoke sensors, heat sensors, gas sensors, etc. Equipment to carry out testing of different types of hazard detector is already available worldwide, and a well-known brand is ‘SOLO’ test equipment. In the past, each hazard sensor has often been housed in its own separate hazard detector, and test equipment to execute the testing of such detectors has mainly used a single test stimulus to activate the sensor concerned, e.g. a heat source is used in an item of test equipment to test a heat detector, etc. The test stimulus (heat, in this example) is designed to replicate the hazard in a non-hazardous fashion, so that the correct operation of the detector and/or the system can be verified without the risk of duplicating the real hazard (e.g. a real fire).
In the case of heat detectors (or any fire detector incorporating heat sensors), a common test method involves blowing hot air from an electrical heating element. The draught of hot air is typically directed at the heat detector, or even just at the heat sensor itself, causing its temperature to rise and the operation of the sensor, the detector, and even the complete cause and effect program of the fire detection system to be checked.
In the case of smoke detectors (or fire detectors which incorporate smoke sensors), a common test medium is an aerosol ‘smoke’ which simulates real smoke. It can be deployed from an aerosol can into the detector, often using a special dispensing tool, so that the operation of the smoke detector and its role within the fire detection system is checked. Also, the test stimulus should be introduced into the hazard detector from outside it, i.e. from the surrounding air, so as to ensure that the entry path to the sensor is not blocked in any way, impeding the ability of the detector to react properly to the hazard.
Functional testing of fire detectors in such a manner as described above is well approved and respected as a good and necessary test of the functioning of a hazard detector. This is widely enough accepted that it is now enshrined in international test standards and codes for the maintenance of fire detectors in various regions of the world (e.g. NFPA 72 in the USA, BS5839 Pt 1 in the UK, etc.).
By contrast, other test methods which do not include the application of a stimulus to the sensors from outside the detector are not widely approved, and indeed are actively prohibited by some test standards. These methods include testing using a magnet which is held close to the detector body, closing a reed switch internally to complete an electrical circuit which indicates an alarm state, or testing a detector for function by means of its internal electronic behaviour only, often done remotely from the control and indicating equipment to which the detector is connected. These methods are not deemed to be sufficient to satisfactorily test the entire operation of the detection device. For example, it may be possible for a hazard detector to have a protective dust cover installed over it, thereby preventing the products of a real hazard from entering its sensors, and yet electrically it may appear to be fully functional and capable of indicating an alarm. Clearly, in this scenario, the ‘electronic only’ test is inadequate since a real hazard would not be detected in such a case, although the test itself may have been apparently passed.
In the case of carbon monoxide detectors (or any detector which incorporates carbon monoxide sensors) a common test method is to introduce small quantities of carbon monoxide to the detector under test. Alternative methods for testing CO sensors within detectors have been known to use other gases such as hydrogen, but the sensors are known to have variable cross-sensitivities to these, and this does not represent a true test that a CO sensor responds to the actual gas which it is intended to detect. Furthermore, the use of a highly flammable gas (such as hydrogen) is not advisable in close proximity to live electrical circuits.
Increasingly, hazard detectors have more than one sensor within them, thereby detecting the hazard using more than one means. The information gathered from multiple sensors can lead to increased efficiency and enhanced speed of reaction in hazard detection. This can protect life and property better and also reduce unwanted alarms.
In the case of fire detectors, for example, a combination of smoke, heat and gas sensors can be packaged together in a single detector. With such an arrangement, the decision concerning the presence of a hazard can be made more effectively, and preferably sooner, and can avoid unnecessary alarm signals for non-threatening hazard stimuli. For example, for fire detection, the presence of smoke alone may not be an indicator of a real fire (e.g. cigarette smoke may be present, although the threat of fire is not sufficiently high to raise an alarm), but the added presence of a sudden increase in temperature and/or the presence of a rising level of combustion gas (i.e. a gas produced as a result of combustion) indicates a much higher probability of a real fire. The gas and/or the heat may even be present before much smoke is prevalent, so by also sensing the gas and/or heat, an earlier alarm could be raised compared to a detector which was only able to sense smoke, for example.
A suitable analysis of the output of multiple sensors can also serve to assist in the reduction of false (or unwanted) alarms by determining which sensors are activated and making a more informed decision to raise the alarm. For example, false alarms are a major concern for the UK fire industry, and so the installation of more multisensor type fire detectors to reduce false alarms is becoming widespread. In addition, there are many ways to interpret the readings of more than one sensor within a detector. The output from multiple sensors can be combined in such a fashion so as to produce a more intelligent response. Software algorithms can be employed either within the detector itself or within the system's control and indicating equipment in order to determine when and if the alarm signal should be raised.
Individual sensors within a multisensor detector may also be disabled or partially disabled (by reducing their sensitivity) to eliminate the risk of false alarms, particularly at certain times of the day or night. For example, in a combined smoke/heat detector, the smoke sensor may be disabled (or set to lower sensitivity) during the hours the building is occupied to avoid false alarms from, say, cigarette smoke. Whenever the building is unoccupied, however, for example at weekends and at night, a greater degree of protection against fire can be enabled by fully activating both the heat and the smoke sensors at higher sensitivity. Utilising an intelligent algorithm can further enhance this. The end result is that a fire can be detected at the earliest possible moment without the risk of an increase in false alarms.
Given the many advanced operating features and the possible combinations within multisensor detectors, testing them is challenging. The use of a safe, clean and environmentally friendly technique is paramount, and so the challenge of testing is made more acute. The generation of real hazard stimuli is not conducive to safe and clean execution and potentially risks harming the future integrity of the hazard detector itself and may even present risks to users of the test medium. The use of simulated hazard stimuli is therefore considered to be the most appropriate means of testing.
With multiple sensors, a single surrogate stimulus (which is intended to simulate the presence of just one of the detectable signs of the hazard) may not be sufficient for the detector to determine that an alarm signal should be raised. Hence, traditional testing techniques used for either a smoke or heat detector may not be sufficient to fully test a multisensor detector. This includes the use of synthetic or simulated smoke aerosols deployed from aerosol cans for the testing of smoke detectors.
In order to test a multisensor hazard detector, it may be possible to operate it in a special test mode, whereby the detector is not operating all its sensors in the usual combined fashion, but in such a manner as to differentiate the responses of each sensor. It may even be the case that such a test mode permits the proper evaluation of the performance of any individual sensor within the detector independent of the other sensors. During testing in this mode, the activation of individual sensors could be signified by the detector's own indicator (e.g. LED) or it may be indicated at the control and indicating equipment to which the detector is connected. In such a test mode, it would be possible to use just one test stimulus at a time for testing. However, in the case that a test mode of this nature is not available or desirable, it may be necessary to use more than one stimulus at the same time. Then, the combined effect of more than one stimulus on the detector simultaneously would be required to activate the detector during a test. Testing such a detector with the normal detection algorithms running (either within the detector itself or within the control and indicating equipment, to which the detector is connected) implies that the detector will only indicate an alarm signal when the combination of stimuli meet the criteria for a real hazard. However, it may be possible to meet these criteria while only activating some of the total number of sensors, and so it can not be seen as a thorough test, as the alarm state may have been reached without the requirement for a particular sensor to respond. This leaves open the possibility that this sensor may indeed not be working. On the other hand, other algorithms may require a response from all sensors within a multisensor detector before an alarm signal is raised, but since it can not easily be determined in a maintenance setting precisely how the detection system operates, the preferred method of maintenance would be to use a test mode as described above.
It is important that each individual sensor within a multisensor detector should be tested for function in its own right, so that when the detector is configured for any mode of operation (utilising some or all of those sensors), it may be able to be relied upon to work correctly. It would be of little use, for example, in a combined smoke and heat detector, to only test the function of the smoke sensor. If that detector were then configured in a mode which relied heavily (or even solely) on the heat sensor, then its correct operation would not have been properly validated.
A far better test would be to introduce multiple test media to the detector to perform functional tests on all the sensors within that detector. That way, any single sensor (or combination of sensors) which is then utilised by the detector in a real life hazard protection situation can be relied upon to react to the hazard correctly.